Is autism more common in children with Down syndrome? Is there any understanding why it might be more common in children with Ds than in the population at large?

Autism occurs in 1 in 110 people according to the latest statistics from the Center for Disease Control and Prevention. Thus, approximately 1% of the population has an autism spectrum disorder. The current estimate of autism in people with Down syndrome is 7-10%. As such, autism does appear to occur much more frequently in people with Down syndrome than in the population at large.

However, this should be evaluated very carefully. First, there are few studies to evaluate this trend. In addition, the measures utilized to provide diagnostic clarification are not normalized or standardized for use on this population. In addition, results were found to be somewhat inconclusive in some studies. Finally, making clear diagnostic distinction is difficult, particularly in individuals with various levels of cognitive impairment.

Oftentimes there is a great deal of overlap between particular diagnoses. For example, individuals with Down syndrome have some level of cognitive impairment. Those individuals with cognitive impairments will be certain to show some delays in language development, social interactions that are young for their age, and a variety of repetitive or self-injurious behaviors. As such, it can be difficult to distinguish to what to attribute the characteristics. In general, a good rule of thumb is that a diagnosis is not added unless it is clear that the symptoms present are over and above those that would be expected from the primary diagnosis. So, in the case of autism and Down syndrome, having some repetitive rocking behavior, though also a symptom of autism, would not be added unless this repetitive behavior occurred to the exclusion or in preference of social or other interactions. In addition, having a speech and language delay that is consistent with the delay expected with the general cognitive delays of the individual in Down syndrome would not be considered a symptom of autism unless the language was also unusual with respect to the pragmatics or functional use of language as in autism. It is possible that autism is overidentified in individuals with Down syndrome due to this common misunderstanding--the presence of characteristics similar to those in autism do not equate to the need for diagnosis of autism.

On the other hand, the diagnostic picture is indeed complicated in that it is also clear that there is an increased risk of autism in a variety of genetic disorders beyond Down syndrome inclusive of Fragile X syndrome and Tuberous Sclerosis. In addition, we know that there are several factors that lead to an increased risk of autism in people with Down syndrome and others:

Male vs. female

History of infantile spasms

Family history of autism

Lower cognitive ability

In that a full understanding of the cause of autism is not yet known, it is difficult to determine why this occurs. What we do know is that the determination of this diagnosis can be quite difficult and we ultimately do not yet have adequate tools to either identify or distinguish autism from other disorders such as Down syndrome. Unlike Down syndrome which has been identified and studied since the 1800s, autism is a relatively new diagnostic category, first identified in 1943 and not intensively studied until the 1980s. There is still much that we do not yet know about this disorder. Until we do, it will make a full understanding of the dual diagnosis with other disorders also difficult.